1. Field of the Invention
The present invention relates to a process for producing a polyester container comprising the step of stretch blow molding, which provides a transparent container having high strength.
2. Description of the Related Art
Polyethylene terephthalate (PET), which is a representative polyester resin, is usually molded into a container by the biaxial stretch blow molding method, and has been widely used as a material for containers because it provides excellent transparency and mechanical strength as well as a relatively good gas barrier.
Meanwhile, in recent years, environmental pollution problems have become important, and the utilization of materials compatible with the environment, resource saving, energy saving, recycling, etc., have been extensively advocated. The development of a container having a thin wall thickness through a reduction in the amount of resin is desired from the view point of resource savings. A significant reduction in the amount of the PET resin, however, causes a reduction of the strength of the container.
Although an increase in the molecular weight of the PET resin is thought to be one of the means for increasing the strength of the container, a mere increase in the molecular weight brings about an unsatisfactory effect with respect to increasing the strength and, what is worse, the melt viscosity becomes so high that the productivity becomes low in the injection step for forming a preform.
WO91/02111 (published on Feb. 21, 1991), the corresponding European Patent Publication-A No. 0491947 (published on Jul. 1, 1992) and the corresponding U.S. Pat. No. 5,162,091 (published on Nov. 10, 1992, Assignee: Kao Corp.) disclose additive compounds for lowering the melt viscosity of PET resins and polyester fiber prepared utilizing the resin composition comprising a PET resin and an additive compound. Although the melt viscosity of the resin composition is reduced by the use of the additive compound, the strength of the fiber prepared with the resin composition containing the additive compound is the same as that of the fiber prepared with the resin composition containing no additive compounds.